(a) Field of the Invention:
The present invention relates to an optical measuring device of film thickness which is capable of measuring film thickness without destroying or contacting measured films.
(b) Description of the Prior Art:
When spectral reflectance of a single-layer transparent thin optical film is measured with a spectral reflectance measuring device, reflectance R shows variations in conjunction with film thickness due to the effect of interference between the lights reflected by the front and rear surfaces of the thin film as wavelength .lambda. of measuring light varies as illustrated in FIG. 1. When reflectance R of a single-layer film has two or more local maxima or minima and a measuring light is incident perpendicularly on the thin film, it is known that thickness d of the thin film is given by the following formula: ##EQU1## wherein the reference symbol N represents number of local maxima or minima existing between a wavelength .lambda..sub.a at which a local maximum or minimum exists and another wave length .lambda..sub.b at which another local maximum or minimum exists, the reference symbols n.sub.1 (.lambda..sub.a) designates refractive index of a substance constituting the thin film at the wavelength .lambda..sub.a and the reference symbol n.sub.1 (.lambda..sub.b) denotes refractive index of the substance constituting the thin film at the wavelength .lambda..sub.b. In case of a multi-layer film wherein said film is composed of a plural number of layers laminated on a substrate and reflectance is measured as a total sum of interference of lights reflected by the individual layers, however, it is impossible to determine thickness values of the individual layers at the same time by the method to detect the wavelengths at which reflectance is maximum or minimum.
In view of this fact, there was contrived a method to determine thickness values of individual layers of a multi-layer film by calculating reflectance of the multi-layer film as a function only of film thickness on the basis of known angle of incidence of a measuring light as well as known refractive indices of medium on the side of incidence, a substrate and layers at each of the wavelengths at which reflectance is measured, setting an evaluation function indicating an overall difference between a measured value and theoretical value of reflectance at each wavelength, and determining a combination of film thickness values which minimizes values of the evaluation function. However, such a series of functions generally has a plural number of local solutions, and it is necessary to search for a global solution while preventing the series of the functions from being settled at the local solutions. For this purpose, it is possible to use a global optimization method which is capable of determining thickness values of individual layers of a multi-layer film by permitting finding a global solution even when a plural number of local solutions exist (Refer to Japanese Patent Preliminary Publication No. Sho 63- 32307).
Though the global optimization method permits determining a global solution with the series of the functions which allows multiple local solutions, said method has a defect that the method requires a very long time when it is repeated until the evaluation function has a minimum values after a vicinity of a global solution is reached.